Corn rootworm damage from drought and resistance

A drought does not eliminate the threat of corn rootworm. This summer, corn rootworm (CRW) larval feeding was seen around the Corn Belt including some very dry areas. For example, University of Illinois (UI) entomologists found significant damage at the university’s research center near DeKalb, Ill.

Dry conditions can heighten CRW damage, according to Mike Gray, UI Extension entomologist. “For fields in which drought conditions exist and several nodes of roots have been destroyed, yield losses caused by corn rootworms can be very significant,” Gray states. “No wonder such significant research investment is made to develop products that can be used to effectively manage this impressive insect pest of corn.”

Bt corn injury

Beyond its ability to thrive in drought, the CRW is mounting an impressive attack on popular Bt hybrids expressing Cry3Bb1 toxins. The first documented case of CRW resistance to Bt hybrids was reported last year. And it appears performance problems with Bt hybrids in some continuous corn-on-corn fields have occurred this year.

Seed company entomologists acknowledge there are problems with Bt hybrids. “First, the technology is quite effective on over 99% of the acres it is planted on,” states Dusty Post, Monsanto global insect management lead. “But a limited number of fields show poorer product performance than what we would like to see. These are corn-on-corn acres planted to corn for more than three years. That invites a lot of pressure from insects because they are in areas where they will thrive.”

Monsanto works with customers to help them manage corn-on-corn acres, Post says. When problems appear, the company’s first recommendation is to switch to a corn-soybean rotation, which may not be economically viable.

Another recommendation is to use a dual-mode-of-action product like SmartStax. SmartStax continues to perform well in high CRW-pressure fields, Post states. A third option is to stick with a single-mode-of-action hybrid and add a soil-applied insecticide and other products like a foliar spray to reduce next year’s crop of beetles.

Insect vs. weed resistance

Insect resistance may sound similar to weed resistance, but Post reports “it is actually quite different. A single resistant weed will produce millions of seeds, which can lay dormant for years. That’s not true of bugs, which are on a one-year cycle. They lay their eggs and if they don’t hatch, they are gone.”

Seeds also can hitch a ride on equipment and easily contaminate other fields, she says. While a few bugs do land on equipment, it is not to the same extent as seed. “Corn rootworm is a fairly contained bug with a field-by-field resistance,” she explains. “If you rotate to soybeans, it breaks the cycle.” There are CRW variants that lay eggs in soybeans, however. These variants require multiple modes of action to control.

And finally, “refuges do work with insects, but do not work with weeds,” Post maintains. “Compliance for planting refuges is important.”

She adds that use of refuge-in-a-bag products like Monsanto’s RIB Complete meets the grower’s refuge requirements.